Draft: Issue #7898: Color picker "RGB (%)' and 'RGB (0..255)' pick in sRGB

This patch attempts to resolve issue #7898 (closed). It imports the current color profile from the image to the ColorFrame widget, and uses it in the babl conversions (as long as it's an RGB color profile). This is done by consolidating the Pixel, RGB % and RGB 0...255 switch cases into one, then using conditional statements to determine the final format (0...255 is U8, while % is FLOAT).

The current profile (if set) is now shown in the GimpColorFrame. The GimpColorArea has also been given access to use the profile so that its "out-of-gamut" indicators are more accurate.

Edit: I need to redo this one since !791 (merged) changes the assumptions it's built on. Hopefully better!